[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Respiratory](https://durgical.com/en/physiology/#s-resp) › Airway resistance [Physiology](https://durgical.com/en/physiology/) · [Respiratory](https://durgical.com/en/physiology/#s-resp) # Airway resistance - Concept - Exam priority: medium The narrower a tube, the harder it is to push air through. Halving the radius multiplies resistance by 16. Updated 2026-11-01 Also called: Poiseuille's law in the airways, Bronchoconstriction **See it in 3D:** [Trachea](https://durgical.com/en/anatomy/trachea/) · [Main bronchi](https://durgical.com/en/anatomy/right-main-bronchus/) · [Lobar bronchi](https://durgical.com/en/anatomy/lobar-bronchus/) · [Segmental bronchi](https://durgical.com/en/anatomy/segmental-bronchus/) ## Key points - R ∝ 1/r⁴: half the radius gives 16 times the resistance. - The main site of resistance is the medium-sized bronchi (up to about the 7th generation). - Sympathetic (β2): bronchodilation. Parasympathetic (vagus, M3): bronchoconstriction. - Higher lung volume, lower resistance: radial traction pulls the airways open. ## What it is Airway resistance is the opposition to airflow; by Poiseuille's law it depends on the radius to the fourth power. Most resistance sits in the medium-sized bronchi, not the bronchioles, because bronchioles are so numerous and in parallel. Smooth muscle tone, mucus, edema and lung volume change it. ## How it works, step by step - In the trachea and large bronchi flow is fast and turbulent. - In the medium bronchi total cross-section is still small, so resistance concentrates there. - Further down, branches multiply, total area grows and the resistance of each level falls. - During a forced blow, positive pleural pressure squeezes the cartilage-free small airways and limits flow. ## Equations **Airflow**Flow = ΔP / R **ΔP** mouth-to-alveolus pressure difference **R** airway resistance **Poiseuille's law**R = 8 η l / (π r4) **η** gas viscosity **l** tube length **r** radius Holds for laminar flow; in the trachea flow is turbulent and resistance is even higher. ## Regulation - *−* **Sympathetic / epinephrine (β2)**: Relaxes bronchial smooth muscle: resistance falls. - *+* **Vagus / acetylcholine (M3)**: Contracts smooth muscle and increases mucus. - *+* **Histamine, leukotrienes, irritants**: Bronchoconstriction and mucosal edema (asthma, allergy). - *−* **High lung volume**: Radial traction from the parenchyma opens the airways. ## Clinical notes - Asthma: bronchospasm, edema and mucus shrink the radius; wheeze and prolonged expiration. - Wheeze (intrathoracic airways) and stridor (extrathoracic airway) come from turbulent flow through a narrowed airway. ## Test yourself Think of the answer, then tap the question to see it. ## If asthma narrows a bronchus's radius by 50%, how much does its resistance rise? About 16-fold (2⁴), because resistance depends on the radius to the fourth power. ## Why do the bronchioles, though so thin, add little total resistance? There are thousands in parallel: their total cross-sectional area is huge and their combined resistance is low. ## Why does an asthmatic breathe at high lung volumes? At higher volume the parenchyma pulls the airways open, lowering resistance, even though it costs more elastic work. [Add to my cards (3)](https://durgical.app/estudiar/?add=/en/physiology/airway-resistance/) ## See it in 3D The structures of Durgical's 3D anatomy where it happens. - [Trachea](https://durgical.com/en/anatomy/trachea/) - [Main bronchi](https://durgical.com/en/anatomy/right-main-bronchus/) - [Lobar bronchi](https://durgical.com/en/anatomy/lobar-bronchus/) - [Segmental bronchi](https://durgical.com/en/anatomy/segmental-bronchus/) ## Related topics - [Obstructive and restrictive patterns, and flow–volume loops](https://durgical.com/en/physiology/obstructive-and-restrictive-patterns-and-flow-volume-loops/) - [Lung and chest wall compliance](https://durgical.com/en/physiology/lung-and-chest-wall-compliance/) - [The work of breathing](https://durgical.com/en/physiology/the-work-of-breathing/) - [Mechanics of breathing: pressures and muscles](https://durgical.com/en/physiology/mechanics-of-breathing-pressures-and-muscles/) - [Airway defenses, cough and mucociliary clearance](https://durgical.com/en/physiology/airway-defenses-cough-and-mucociliary-clearance/) ## Related conditions - [Asthma](https://durgical.com/en/conditions/asthma/) - [Chronic obstructive pulmonary disease (COPD)](https://durgical.com/en/conditions/chronic-obstructive-pulmonary-disease/) - [Bronchiectasis](https://durgical.com/en/conditions/bronchiectasis/) [← All of physiology](https://durgical.com/en/physiology/) Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given. Sources: [22.3 The Process of Breathing](https://openstax.org/books/anatomy-and-physiology-2e/pages/22-3-the-process-of-breathing) (OpenStax (Rice University), 2022); [Asthma](https://www.ncbi.nlm.nih.gov/books/NBK430901/) (StatPearls (NCBI Bookshelf), 2024); [Anatomy, Thorax, Tracheobronchial Tree](https://www.ncbi.nlm.nih.gov/books/NBK556044/) (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 38). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/physiology/airway-resistance/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/airway-resistance/